The present invention relates to a diode attenuator for high frequencies and, more particularly, to an improvement in a voltage controlled diode attenuator.
A prior art diode attenuator, of the kind described, is constructed to apply a predetermined dc bias voltage via a choke coil to a series connection of two diodes, and to apply a variable bias control voltage via another choke coil to one of the diodes. The control voltage is varied to control the attenuation of a high frequency signal which flows through the other of the two diodes. Due to the predetermined voltage applied between the two diodes, the input impedance of the attenuator advantageously remains constant despite changes in the attenuation of the diode caused by the variable control voltage, as is well known in the art.
A problem encountered with such a prior art attenuator is that the choke coils included therein make it difficult to set up a wide and usable frequency band attenuator or to provide the attenuator in an integrated circuit. Although the use of resistors in place of the choke coils may be contemplated for biasing purpose, it brings about another problem since a change in the control voltage causes fluctuations in the voltage drops of the resistors. This fluctuation prevents the voltage applied between the two diodes from setting at a predetermined level, resulting in fluctations of the input impedance of the attenuator.